Process for making an electrostatic recording medium

ABSTRACT

In an improved process for making an electrostatic recording medium, the exposed surface of the dielectric recording layer formed on a surface of an electrically conductive substrate is subjected to a scraping treatment with a scraper.

United States Patent Shibata et al.

July 1, 1975 PROCESS FOR MAKING AN ELECTROSTATIC RECORDING MEDIUM Inventors: Kazuo Shibata; Takao Matsushita,

both of Hyogo, Japan Kanzaki Paper Mfg. Co. Ltd., Tokyo, Japan Filed: Mar. 16, 1973 Appl. No.: 342,266

Assignee:

Foreign Application Priority Data Mar, 18, 1972 Japan 47/27852 US. Cl 427/121; 427/358 Int. Cl. H0111 1/00 FieldofSearch 117/111 R, 111 F, 161 R,

117/20L208. 155 UA, 155 R, 155 L [56] References Cited UNITED STATES PATENTS 3,000,760 9/1961 Greiller .4 117/111 F 3.097964 7/1963 Stowell r r 1 r r 117/155 L 3,634,135 1/1972 Osaka 4. 117/221 Primary Examiner-Mayer Weinblatt Attorney Agent, or FirmCushman. Darby & Cushman 1 1 ABSTRACT In an improved process for making an electrostatic re cording medium, the exposed surface of the dielectric recording layer formed on a surface of an electrically conductive substrate is subjected to a scraping treatment with a scraper.

6 Claims, 13 Drawing Figures PROCESS FOR MAKING AN ELECTROSTATIC RECORDING MEDIUM BACKGROUND OF THE INVENTION This invention relates to an improved process for making an electrostatic recording medium. This invention more particularly relates to an improved process for making an electrostatic recording medium providing satisfactory image-resolvability and free from uneven electric charging.

It is known to make electrostatic recording media which comprise an electrically conductive substrate and a dielectric recording layer on a surface of said electrically conductive layer. Generally, the dielectric recording layer is made of an insulating resin such as vinyl acetate resin, vinyl chloride resin, silicone resinepoxy resin, vinyl acetate-methacrylate copolymer resin or vinyl acetate-styrene copolymer resin. An attempt was made to improve such electrostatic record ing media by precluding the possibility of errors in printing and of uneveness in record. For example, Japanese Pat. No. 8038 of l97l discloses to calender a paper substrate either in advance or after the formation of a dielectric layer thereon to achieve a smoothing ef fect. With such treatment by pressing, however, since the recording medium itself is compressed, the stiffness is descreased while the bulk density is increased. As a result, various drawbacks are caused, including detrac tion from the natural appearance of paper with the concurrent loss of flexual regidity, unevenness in record leading to a decrease in image-resolvability, and a decrease in the amount of electric charge resulting in a decrease in image density. Other various attemps have been made to impart natural appearance or writability to the electrostatic recording medium. Among them there is Japanese Utility Model Publication No. 20,592 of 1963 which discloses to add organic powder such as starch and cellulose powder or inorganic powder such as barium sulfate to the insulating resin. US. Pat. Nov 3,097,964 discloses to add mechanically pulverized polymer to the insulating resin layer. US. Pat. No. 3,634,l discloses to include in the insulating resin layer solid polymer particles which are obtained through the utilization of a particular polymerization technique. These electrostatic recording papers have their natural appearance or writability improved since the inclusion of solid particles results in the dielectric layer forming mat finish and a finely roughened surface. Unless such surface-roughening is severely con trolled, however, there inevitably occurs the so-called white-spots which means the discontinuity of the image due to the lack of intimacy of contact with the recording styli.

The primary object of the invention is to provide a new and improved process for making an electrostatic recording medium in which the recording characteristics can be remakably improved without deceasing the stiffness and increasing the bulk density.

Another object of the invention is to provide a new and improved process for making an electrostatic recording medium in which the so-called white spots can be avoided, particularly when applied to electrostatic recording media having a finely roughened suface.

A further object of the invention is provided an electrostatic recording medium having a natural paper appearance and a good writability".

SUMMARY OF THE INVENTION Briefly, the process for making an electrostatic recording medium according to the invention comprises the steps of forming a dried dielectric recording layer on a surface of an electrically conductive substrate. said dielectric recording layer comprising an insulating resin material. and subjecting the exposed surface of said dielectric recording layer to a scraping treatment with scraper means. The scraper means includes an elongated contact area which is in contact with the exposed surface of the dielectric recording layer in a direction transverse to the dielectric recording layer.

The exposed surface of said dielectric recording layer may preferably be scraped by drawing it on said contact area of said scraper with a relatively narrow contact length within the range of about 0.1mm to mm.

The contact surface of scraper means may preferably be formed of a material having a Rockwell hardness within the range of about M70 to M and a coefficient of friction against clean steel within the range of about 0.l to 0.7. It is also preferred that the contact surface of the scraper means is in contact with the exposed surface of the dielectric recording layer under a contact pressure layer than 5Og/cm In many cases one scraping treatment is enough to achieve the objects of the invention but it may be repeated a plurality of times.

In a preferred embodiment of the invention, scraper means comprises an elongated member having an edge to contact the exposed surface of the dielectric record ing layer. The edge is arranged in a direction transverse to the length of the dielectric recording layer.

In another embodiment of the invention, scraper means comprises an elongated member having a contact surface or area and a cooperating member urging the exposed surface of the dielectric recording layer to the contact area of the elongated member. The electrostatic recording medium is drawn between the elongated member and the cooperating member with the exposed surface of the dielectric recording layer being in contact with the contact area of the elongated member.

The electrostatic recording medium produced according to the invention comprises an electrically conductive substrate and a dried dielectric recording layer formed on a surface of said electrically conductive substrate. The dielectric recording layer comprises an insulating resin material and has an exposed surface which has had a scraping treatment.

BRIEF DESCRIPTION OF DRAWINGS In the drawings:

FIGS. 1 to 5 illustrate in vertical sectional views five species of scraper means used in the examples of the present invention; and

FIGS. 6 to 13 show enlarged photographs of recorded images in examples of the present invention and in control examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The electrically conductive substrate used for the present invention may be prepared by any conventional method. The substrate may be any of paper, plastic film and synthetic paper treated to have electrical conductivity. The technique for forming a dried dielectric recording layer on a surface of the electrically conductive substrate, per se, may also be conventional. The dielectric recording layer comprises an insulating resin. Among the conventional methods for the preparation of an electrostatic recording medium comprising an electrically conductive substrate and a dried dielectric recording layer on a surface of said electrically conductive substrate. there may be included those disclosed in Japanese Utility Model Publication No. 20,592 of I963, US. Pat. No. 3.097.964 and US. Pat. No. 3,634,l35.

According to the invention. the exposed surface of the dielectric recording layer thus formed on the substrate is then subjected to a scraping treatment. Preferably the scraping treatment is applied uniformly throughout the whole width of the dielectric recording layer. but it may be applied partly alone if so required. The scraping treatment is carried out with use of scraper means.

Some different species of scraper means are illustrated in FIGS. 1 to of the drawings. Throughout FIGS. 1 to 5, like reference numerals indicate like parts, respectively. Referring first to FIG. 1, a scraper 21 in the form of an elongated blade is illustrated in contact with a moving electrostatic recording medium generally indicated with the reference numeral 22. The electrostatic recording medium 22 comprises an electrically conductive substrate 23 and a dielectric record ing layer 24 formed on a surface of the electrically con ductive substrate 23. The scraper 21 has at its one end an elongated contact edge which is in contact with the exposed surface of the dielectric layer 24 of the moving electrostatic recording medium 22. The contact edge 25 has a semi-circular cross sectional shape.

In the embodiment illustrated in FIG. 2, the scraper 21 is formed in a bar having a circular cross section. The contact edge 25 extends along a top generatrix.

FIG. 3 illustrates a scraper 21 in the shape ofa triangle bar with a contact edge 25.

FIG. 4 illustrates a scraper 21 of another shape. namely, having a contact edge 25 in a trapezoid shape. The exposed surface of the dielectric recording layer 24 is in contact with the top area 250 of the edge portion 25 and the shoulder portions 25b and 25c before and after the top area 25a.

Throughout the embodiments illustrated in FIGS. 1 to 4. the scraping treatment is carried out by drawing the electrostatic recording medium 22 on the contact edge 25 of the scraper. The contact edge 25 is arranged in a direction transverse e.g. substantially perpendicular to the moving direction of the electrostatic recording medium 22.

FIG. 5 illustrates a further embodiment of the invention in which scraper means generally indicated with the reference numeral 21 comprises an elongated plate 21a having a flat contact surface 25 and a cooperating member 21b in the form of a blade similar to the scraper illustrated in FIG. 1. In this embodiment, the exposed surface of the dielectric recording layer 24 of the electrostatic recording medium 22 is in contact with the flat surface 25 of the plate 21a with the bottom edge 211)!) of the cooperating member 21b being in contact under pressure with the other surface 230 of the substrate 23. The exposed surface of the dielectric recording layer 24 is subjected to a scraping treatment by drawing the electrostatic recording medium between the flat plate 2la and the cooperating member 21b. The cooperating member 21b is arranged in adirection transverse, e.g. substantially perpendicular, to the moving direction of the electrostatic recording medium 22.

Throughout all the embodiments of the invention illustrated in FIGS. 1 to S, the exposed surface of the dielectric recording layer 24 is in continuous contact with the contact edge or area 25 of scraper means 21. The contact length in the moving direction of the electrostatic recording medium is as relatively short as within the range of 0. lmm to 100 mm.

The contact edge or area 25 of scraping means 21 may preferably be made of a material having a Rockwell hardness within the range of about M to M I20. The coefficient of friction of the contact edge or area 25 against clean steel may preferably be within the range of about 0.1 to 0.7. Among the materials useful for forming the contact edge or area there may be included synthetic resins such as acrylic resin. ABS resin, vinyl chloride resin and polyester resin, hard rubber, wood and metals. From the view point of the coefficient of friction, acrylic resin of ABS resin is most preferred.

Further. even when a material which has not so high a coefficient of friction used, a good result can be obtained by increasing the contact pressure of the blade or rod or by increasing the number of times of the scraping treatment.

The contact pressure between scraper means and the dielectric recording layer is preferably at least SOg/cm? It is, however, neither necessary nor desirable to apply such a large contact pressure as in case of the calender treatment which will cause to destroy the recording layer. Usually, the upper limit of the contact pressure would be 3OOg/cm or lower. preferably, lSOg/cm" or lower.

After the scraping treatment according to the invention, the surface of the recording medium thus treated may sometimes be observed to have powder most of which is believed to have arisen from the wear of the solid body. Such powder may be removed by brushing and/or suction. There may be worries that the scraping treatment will be attended with the development of frictional charge. As we have ascertained, however, such frictional charge due to the scraping treatment, contrary to our apprehensions, was almost unnoticeable or even if it was noticeable it was not so serious as to affect the recordability. But in order to eliminate any effect of frictional charge due to scraping treatment on the recordability. there may be contemplate the use of a suitably selected resin which will neutratize the frictional charge or the removal of frictional charge by an eliminator after the scraping treatment. The electrostatic recording media produced according to the present invention are superior in that they possess satisfactory stiffness and bulk density and are free from troubles such as the so-called white spots due to uneven electric charge. The function of the process according to the invention has not yet been made clear. We have, however, ascertain that the effects and advantages obtained according to the invention are those which can never be expected when smoothing treatment such as a conventional calendering treatment is employed.

The invention will now be described more in detail with reference to the examples thereof. In these examples parts are represented by weight, unless otherwise indicated.

EXAMPLE I Base paper of 50 g/m consisting of l% hard wood bleached kraft pulp was size-pressed with a aque ous solution of sodium alginate to prepare an electri cally conductive paper substrate. One surface of said paper substrate was coated with a methyl ethyl ketone solution of 30% concentration containing 60 parts of vinyl chloride-vinyl acetate copolymer resin (70:30) and 40 parts of calcium carbonate by a roll coater in such an amount as to provide a dry weight of fig/m and was then dryed, thus preparing an electrostatic record ing paper. The resin-coated surface of the electrostatic recording paper was subjected to the scraping treatment with an acrylic resin scraper in the form ofa blade in a state of contact as shown in FIG. 1. The scraper had a thickness of 25mm and a contact edge of a semicircular cross section of which the radius of the curvature ws 12.5mm. The scraper was made of acrylic resin having a Rockwell hardness of M 100 and a coefficient of friction of 0.45 against clean steel. The scraping treatment was carried out under a contact pressure of 62g/cm'-. The contact length on the contract edge was mm. The electrostatic recording paper thus treated possessed satisfactory stiffness and bulk density as well as in ordinary paper. After it had a signal charge corresponding to an original applied thereto at an applied voltage of 500V by an electrostatic facsimile apparatus of the multi-stylus type having a line density of 8l/mm, it was powder-developed. As a result, there was obtained an image having satisfactory dot reproducibility and no unevenness in electric charging. An enlarged photograph, magnified 50 times, of this recorded image is shown as FIG. 6.

As a control example, a similar recording was made on an electrostatic recording paper which was treated in the same manner as the one described above except that it was not subjected to the scraping treatment. The white spot phenomenon was remarkable. An enlarged photograph. magnified 50 times, of this recorded image is shown as FIG. 7.

A comparison between FIGS. 6 and 7 reveals that in FIG. 6 there is no unevenness in electric charging (i.e. the dots have the same size) and there is no white spot whereas in FIG. 7 there are many white spots where dots have failed to appear.

EXAMPLE 2 Base paper of 50 g/m consisting of [00% hardwood bleached kraft pulp was coated with a mixture of 50 parts of 50% ECR-34 (cationic surfactant produced by Dow Chemical, USA), and 50 parts of 10% polyvinyl alcohol solution by an air knife coater to provide a 6 ,u thick low-resistance layer thereon. The latter layer was then coated with isopropyl alcohol solution of 30% concentration containing 70 parts of polyvinyl butyral resin and 30 parts of barium sulfate by a roll coater in such an amount as to provide a dry weight of 6 g/m thereby preparing an electrostatic recording paper. The resin-coated surface of this electrostatic recording paper was subjected to the scraping treatment with a scraper rod having a diameter of 20 mm in a state of contact as shown in FIG. 2. The scraper rod was made of ABS resin having a coefficient of friction of 0.40 against clean steel and a Rockwell hardness of M95.

The scraping treatment was carried out under a Contact pressure of l25 g/cm The contact length was 20 mm in arc. The scraping treatment was repeated twice. The electrostatic recording paper thus treated possessed good stiffness and bulk density similar to those of ordinary paper. After it had a signal charge corresponding to an original applied thereto at an applied voltage of 400V by an electrostatic facsimile apparatus of the multi-stylus type having a line density of Ell/mm, it was powder-developed. As a result, there was obtained an image having satisfactory dot reproducibility and no unevenness in electric charging. An enlarged photograph, magnified 50 times, of this recorded image is shown as FIG. 8.

As control examples, electostatic recording papers were prepared, one without the scraping treatment, the other with the usual super-calendering in lieu of the scraping treatment. The elctrostatic recording paper which had been calendered turned out to be a nerveless soft paper due to degradation in bulk density and stiffness. Further, the images recorded on the two controls under the same conditions as this Example showed remarkable white spot" phenomena. Enlarged photographs, magnified 50 times, of these recorded images are shown as FIG. 9 (for one not subjected to the scraping treatment) and as FIG. 10 (for one subjected to the calendering treatment).

When FIGS. 8, 9 and 10 are compared with each other, it is seen that in FIG. 8 for the example ofthe invention there is not a single dot which has failed to appear and there is no unevenness in electric charging, whereas in FIG. 9 for the example without the scraping treatment, white spots where dots have failed to appear are clearly seen, and in FIG. 10 for the calendered example, the dots are not of the same size demonstrating the presence of unevenness in electric charging and white spots where dots have failed to appear are clearly seen.

EXAMPLE 3 The paper substrate provided with a low-resistance layer used in Example 2 was coated with a 25% solution of parts of vinyl chloride-vinyl acetate copolymer (:15) and 20 parts of acrylic resin in toluen by a roll coater in such an amount as to provide a dry weight of 4 g/m and was dried, thereby preparing an electrostatic recording paper having a smooth surface. The re sin-coated surface of the electrostatic recording paper was subjected to the scraping treatment with a scraper blade in a state of contact as shown in FIG. 4. The scraper blade was made of ABS resin having a coefficient of friction of 0.40 against clean steel and a Rockwell hardness of M95. The scraper blade has a trapezoid section. The thickness of the scraper blade was 25 mm and the contact edge portion consisted of a top area having a length of 20 mm and a pair of inclined shoulder portions in the opposite sides thereof, each shoulder portion having a length of 5 mm. The total contact length was therefore 30 mm. The scraping treatment was carried out once under the contact pressure of 80 g/cm The electrostatic recording paper thus treated possessed as good bulk density and stiffness as in ordinary paper. After it had an electric charge applied thereto at an applied voltage of 600V by an electrostatic facsimile apparatus of the multistylus type having a line density of 8I/mm, it was powder-developed. As a result, there was obtained an 7 image having satisfactory dot reproducibility and little unevenness in electric charging. An enlarged photograph, magnified 50 times. of this recorded image is shown as FIG. ll.

Further. as control examples, electrostatic recording papers were prepared. one without the scraping treatment of this example. the other with an ordinary supercalendering in lieu of the scraping treatment. and recording was made on them under the same condition as in this example. Each exhibited a conspicuous white spot" phenomenon. Enlarged photographs. magnified 50 times. of these recorded images are shown as FIG. 12 for one not subjected to the scraping treatment and as FIG. I3 for one subjected to the super-calendering treatment.

When FIG. ll, 12 and 13 are compared with each other. it is seen that in FIG. 11 for the example accord ing to the invention there is no white spot" and little unevenness in electric charging whereas in FIG. 12, the so-called white spots" where dots have failed to appear are clearly seen and in FIG. 13, unevenness in electric charging and white spots are conspicuous and. moreover. there is seen a decrease in the image density due to a decrease of electrical charge as a result of the calendering treatment.

What is claimed is:

l. A process for making an electrostatic recording medium comprising the steps of forming a dried dielectric recording layer on a surface of an electrically conductive substrate selected from the group consisting of paper. plastic film and synthetic paper. said dielectric recording layer being essentially formed of an insulating resin material. and scraping the exposed surface of said dried dielectric recording layer by drawing same transversely across an elongated scraping means with contact area having a relatively narrow contact length within the range of about 0.] mm to 100 mm. said contact area having a Rockwell hardness within the range of about M to M and a coefficient of friction against clean steel within the range of about 0.] to 0.7.

2. A process for making an electrostatic recording medium as defined in claim I, in which said Contact area of said scraper means is formed of a member selected from the group consisting of acrylic resin and ABS resin.

3. A process for making an electrostatic recording medium as defined in claim 1, in which said contact area of said scraper means is in contact with said exposed surface of said dielectric recording layer under a contact pressure higher than 50 g/cm 4. A process for making an electrostatic recording medium as defined in claim I, in which the scraping treatment is repeated a plurality of times.

5. A process for making an electrostatic recording medium as defined in claim 1, in which said scraping means comprises as elongated member having an edge to contact said exposed surface of said dielectric recording layer. said edge being arranged in a direction transverse to the length of said dielectric recording layer.

6. A process for making an electrostatic recording medium as defined in claim 1, in which said scraping means comprises an elongated member having a contact area and a cooperating member urging said exposed surface of said dielectric recording layer to said contact surface of said elongated member. and. said electrostatic recording medium is drawn between said elongated member and said cooperating member with said exposed surface of said dielectric recording layer being in contact with said contact area of said elongated member. 

1. A PROCESS FOR MAKING AN ELCTROSTATIC RECORDING MEDIUM COMPRISING THE STEPS OF FORMING A DRIED DIELECTRIC RECORDING LAYER ON A SURFACE OF AN ELECTRICALLY CONDUCTIVE SUBSTRATE SELECTED FROM THE GROUP CONSISTING OF PAPER, PLASTIC FILM AND SYNTHETIC PAPER, SAID EIELECTRIC RECORDING LAYER BEING ESSENTIALLY FORMED OF AN INSULATING RESIN MATERIAL, AND SCRAPING THE EXPOSED SURFACE OF SAID DRIED DIELECTRIC RECORDING LAYER BY DRAWING SAME TRANSVERSELY ACROSS AN ELONGATED SCRAPING MEANS WITH CONTACT AREA HAVING A RELATIVELY NARROW CONTACT LNGTH WITHIN THE RANGE OF ABOUT 0.1 MM TO 100MM, SAID CONTACT AREA HAVING A ROCKWELL HARDNESS WITHIN THE RANGE OF ABOUT M 70 TO M 120 AND A COEFFICIENT OF FRICTION AGAINST CLEAN STEEL WITHIN THE RANGE OF ABOUT 0.1 TO 0.7.
 2. A process for making an electrostatic recording medium as defined in claim 1, in which said contact area of said scraper means is formed of a member selected from the group consisting of acrylic resin and ABS resin.
 3. A process for making an electrostatic recording medium as defined in claim 1, in which said contact area of said scraper means is in contact wIth said exposed surface of said dielectric recording layer under a contact pressure higher than 50 g/cm2.
 4. A process for making an electrostatic recording medium as defined in claim 1, in which the scraping treatment is repeated a plurality of times.
 5. A process for making an electrostatic recording medium as defined in claim 1, in which said scraping means comprises as elongated member having an edge to contact said exposed surface of said dielectric recording layer, said edge being arranged in a direction transverse to the length of said dielectric recording layer.
 6. A process for making an electrostatic recording medium as defined in claim 1, in which said scraping means comprises an elongated member having a contact area and a cooperating member urging said exposed surface of said dielectric recording layer to said contact surface of said elongated member, and, said electrostatic recording medium is drawn between said elongated member and said cooperating member with said exposed surface of said dielectric recording layer being in contact with said contact area of said elongated member. 